This invention relates to a strain relief boot which, when affixed to an electrical cable and connector, will act in a manner that greatly reduces the stresses transmitted from the cable to the electrical contact, thereby preventing wire breakage and will also be flexible enough so as to allow free movement of said cable when in use so that bending and kinking of the cable does not occur at the cable-boot transition point causing wire breakage. The invention is more particularly related to a strain relief boot which may be molded directly onto a prewired electrical connector and cable or as an alternative embodiment may be molded separately for later assembly on the cable and connector at the time they are joined together.
It has been a continuing problem to prevent wire and cable motion and stress from being transmitted to the connector pins when the wires are affixed using solder or by crimping methods and allowing the wires to fatigue and break at that point, at the same time maintaining sufficient flex capabilities so as to not inhibit the normal life span of the cable. Many devices designed to avoid this problem rely upon adding complicated or expensive plastic or metal parts to the connector or imbedding compressible metal clamps within the boot which can be tightened around the cable by pressure upon the outside of the boot (such as that disclosed in U.S. Pat. No. 2,774,948). Prior art strain relief techniques rely upon utilizing a boot of uniform hardness that is tapered from the widest portion at the connector end to the narrowest portion at the cable exit end, the hardness of which is selected as a compromise between that hardness needed to mechanically capture the connector and the softness needed near the exit of the boot to provide the flexibility needed for cable motion. This often results in a heavy, long, or bulky device, too soft for adequate mechanical anchoring on the connector regardless of thickness, but not soft enough to provide flexibility even with severe tapering to nearly the diameter of the enclosed cable. This type of tapered boot then fails to adequately grasp the connector body, which allows separation, and/or rigidly holds the cable, which prevents adequate flexure. Inadequate flexure allows the cable to exceed its minimum bend radius, kink and thus cause cable failure. Tapering as a means for providing increasing flexibility to resist sharp bends in the cable is utilized in the varied strain relieving components of the connectors disclosed in U.S. Pat. Nos. 3,961,833; 3,720,906; 3,093,432; 2,954,541; 2,756,402; 2,032,780 and 1,574,020.
Uniformly spaced multiple slots in the body of the strain relief boot, designed to allow more flexibility, have also been used (U.S. Pat. No. 3,093,432). Tooling cost for this boot is extremely high and when transverse torque is placed on the cable, this slotted type boot will often twist to one side or the other to accommodate the combination of compressing and stretching of the various ribs and slots, thereby causing excessive stress on the both boot and cable. The slotted-type strain relief boot, while allowing better flexibility than the more rigid tapered version, still does not accommodate the stress of cable pull. The jacket of the cable is often stretched just behind the boot when the cable is pulled excessively. When the stress is released, the jacket recovers, which allows a bulge to occur at this point, thus weakening the jacket wall and contributing to further cable failure.
A spiral coil slot molded into the boot instead of spaced multiple slots avoids the above problems of uneven stress, but it does not offer the desired gradually increasing resistance to bending of the cable and boot. It is easier to bend the further it is bent away from parallel.
Therefore, the prior art strain relief boots do not completely and satisfactorily solve all the problems inherent in this cable-connector transition area. Not only do the transverse and axial forces applied to the cable and wires it contains need to be transmitted away from the wire-contact junction, but this has to be done while allowing sufficient flexibility so as to not inhibit the normal life span of the cable.